This invention relates generally to fixed-pitch, screw propellers for marine use and more particularly to correction of rotational speeds of these propellers mounted on the propeller shafts of ships. More specifically, the invention relates to improvement in a method of correcting the rotational speed of such a propeller by adjusting the effective pitch thereof.
As is well known, a fixed-pitch screw propeller installed on a ship is initially suitable for the hull and engine of that ship, but the resistance to its rotation inevitably increases after the elapse of a number of years from the start of its operational service, although there are differences in the degree of this increase. In general, this phenomenon is caused by the combined effect of an increase in the hull frictional resistance due to fouling or roughening of the underwater outer surface of the hull and an increase in the propeller torque due to changes such as fouling of the surfaces of the propeller. As a result, in order to maintain the rotational speed of that propeller exactly as before, a correspondingly larger torque of the main engine, that is, output, becomes necessary.
This so-called "torque rich" phenomenon inevitably occurs to a greater or less degree without exception in a ship fitted with a fixed-pitch screw propeller irrespective of the type of the main engine, as mentioned hereinabove. When it does occur, the mean effective pressure (m.e.p.) and exhaust gas temperature of the main engine become high, especially in a diesel driven ship, and there is a possibility of the engine becoming overloaded. Accordingly, in order to prevent this state, output control becomes unavoidable, and consequently the payability of continuous operation drops remarkably.
In this connection, the magnitude of this "torque richness" may be considered, as a practical value, to be of an order corresponding to from 3 to 4 percent of the initial torque. In most cases, moreover, it may be said that the magnitude of required correction (increase) of the rotational speed is of the order of from 3 to 4 rpm.
The case where, in a ship undergoing sea trials upon being completed and prior to delivery, the torque required for turning the propeller is found to be greater than that expected (torque rich) or, conversely, less than that expected (torque poor) cannot be said to be nonexistent, although it is very rare.
Such a state wherein the required torque is greater or less than the expected torque at the time when the ship is newly built is due principally to a cause such as a miscalculation of the relation between engine power and speed of the ship, unsuitable estimation of the wake coefficient, or unsuitable value of transmitted power. Irrespective of the principal cause, however, it can be stated that the propeller is inappropriate for the hull and the engine at that a pitch which is too large or is too small is indicated.
As measures for overcoming the difficulties due to such a state, for example, a torque rich state, two corrective methods have heretofore been generally practiced. In the first of these methods, the propeller pitch is left as it is, and the blade tips are cut off thereby to reduce the diameter of the propeller. In the second method, the effective pitch of the propeller is appropriately reduced by mechanically imparting a twist to all blades of the propeller by a hot-working process.
In order to carry out a desired correction of the rotational speed by the above first method, however, it is necessary to cut off the blade tips to an appreciable extent and, moreover, to carry out considerable working and shaping operations, whereby much labor and other costs are incurred. In addition, as a result of the reduction in the total blade area, undesirable cavitation readily occurs, and the propeller efficiency decreases. Consequently, even if the adjustment and increase of the propeller rotational speed is thus attained, this method entails undesirable results such as a remarkable drop in the speed of the ship.
In the actual practice of the above second method, various difficulties are encountered. For example, it cannot be carried out in a dock. This method, when carried out, not only entails time, labor, and other high costs but also is accompanied by other serious problems relating to strength and material such as the effects of residual stress due to twist working of the blades.
In view of the above described problems, we have previously proposed a method, as disclosed in the specification of Japanese Pat. Publn. No. 36589/1972, by which the effective pitch of a propeller can be appropriately reduced with almost no lowering of the propeller performance and with the propeller installed on the propeller shaft of a ship. Moreover, this method can be carried out with relative ease and with little labor even with the ship in an afloat state. Accordingly, it is apparent that this proposed method is superior by far to the above described two known corrective methods and has great utilization value.
In this proposed method, however, in order to vary the shape of the blade cross section, the work of cutting the trailing edge of each blade along the blade outline, albeit within a limited range, and at the same time of machining and shaping the blade face or pressure side into a fair curved surface thereby to provide a suitable washback is necessary.